At present regulations on power supply efficiency in various countries are increasingly upgraded to a higher level. For instance, Energy star publishes 80 PLUS standard for multi-output power efficiency of computers and servers. The 80 plus standard has four categories: 80 plus E-star 4.0, 80 plus bronze, 80 plus silver and 80 plus gold. 80 plus gold demands power supply to have efficiency of 87%, 90% and 87% when loading is respectively at 20%, 50% and 100%. As the present power supply of computers and servers generally adopts the structure of PFC+multi-output DC/DC, when the PFC (Power Factor Correction) is at low line, the efficiency usually can reach 94%. To meet the requirement of 80 plus gold, the efficiency of the DC/DC converter at the rear stage has to be greater than 96%. As miniaturization is the prevailing trend for design of electronic devices, a higher power density is required on computer power supply. Hence it is a greater challenge for power supply producers to develop multi-output DC/DC converters with a higher efficiency and greater power density.
A high power density multi-output DC/DC converter has to meet three basic requirements: high efficiency, high voltage stability and small size. The present ATX (Advanced Technology Extended) does not have a special requirement on the issue of size shrinking, hence high efficiency and high voltage stability become the main goals of power supply design. On the design of power supply, hold up time of computer supply is another important design factor. When input power exists, PFC transforms the input power to an adjustable DC voltage (such as 400V). When the input power is absent, the power supply has to maintain output voltage at 20 mS. The 20 mS is the hold up time. During the hold up time, the voltage of 400V output from the PFC continuously drops until reaching a minimum voltage, then DC/DC turns off, and the output voltage vanishes. During the hold up time, output has to be maintained stable. Hence DC/DC must be operable at a selected input voltage range. The selected input voltage range relates to capacitor capacitance and power, and can be indicated by an equation as follow:
  E  =            1      2        ⁢                  C        ⁡                  (                                    V              bus              2                        -                          V              min              2                                )                    20      
Based on the equation above, the capacitor capacitance is in inverse proportion with the voltage range. Hence by enlarging the input voltage range, the size of the capacitor can be reduced to increase the power density. But increasing the input voltage range makes topology selection and circuit design more difficult. This is an issue yet to be fully resolved.
The key factor of increasing efficiency is to restrain switching loss of the converter. The conventional converter performs switching at a high current or voltage (hard switching) and generates a great switching loss. An improved approach has been developed to perform switching at a lower current or voltage (soft switching), or even at zero current or zero voltage. At present, there is a number of circuit topology to provide a higher efficiency such as active clamp forward (ACF) circuit, LLC circuit and dual active bridge (DAB) circuit. FIG. 1 illustrates a typical ACF circuit. Its structure and operational principle are known in the art, thus details are omitted. The ACF circuit can partially or fully realize ZVS (zero voltage switching) of a primary switch, hence has a higher efficiency. Meanwhile, the magnetic core of the transformer operates in the first and third quadrants, hence its utilization is higher. However, ACF circuit has a number of inherent drawbacks, such as hard switching off causes a greater switching loss, an extra inductor has to be added and a smaller magnetizing inductance is needed to fully realize the ZVS. Increasing the inductance reduces the equivalent duty cycle, lowering the magnetizing inductance increases the magnetizing current and results in a greater copper loss and conduction loss. It also has cross regulation problem. Hence the conventional PWM (pulse-width modulation) controlled ACF is difficult to meet the duty requirement of a higher efficiency and a greater voltage range at the same time. As a result, the ACF structure generally does not provide a high efficiency as desired. But due to it can realize ZVS during turn-on, its efficiency is higher than the general PWM structures such as double-transistor forward, half-bridge circuit and the like. On-semi Co. provides an ACF design with 250W ATX power to meet 80 plus E-star 4.0. The power can achieve an efficiency greater than 80% at the loading conditions of 20%, 50% and 100%.
At present, the rear stage DC/DC with most promising application prospect is LLC circuit (referring to FIG. 2). The LLC circuit is a resonant circuit consisting of two L (inductors) and one C (capacitor). Through a switch 301, the period of an input power flowing to a LLC circuit 302 can be controlled. The input power has gain through the LLC circuit 302, then is sent through a transformer 303 to the secondary side thereof to be output. The primary side of the transformer has a control chip 304 to generate a control pulse to regulate the switching period of the switch 301 through a driver 305. FIG. 3 shows the waveforms at various nodes of the circuit depicted in FIG. 2, and FIG. 4 shows the resonant characteristics curves of the circuit. The LLC circuit provides many advantages, such as switching at zero voltage, lower turn-off loss, wide operation voltage range without sacrificing efficiency at normal condition, lower PFC capacitor capacitance, smaller size and higher power density.
The DAB circuit for a greater power DC/DC can achieve a higher power density. FIG. 5 illustrates a DAB circuit which includes two sets of full bridge switches 301, an inductor and a transformer 303 to isolate power. It also controls operation of the switch 301 through a control chip 304 and a driver 305. FIG. 6 shows the power waveforms at various nodes of the circuit depicted in FIG. 5. The drawing shows that the phase of the bridge of the primary side and that of the secondary side in FIG. 5 has a phase shifting angle Φ. By changing the phase shifting angle and the switching frequency, output gain of the circuit can be regulated. Moreover, by controlling the phase of the primary side and the secondary side, ZVS of all switches can be accomplished and output power also can be regulated. It has the characteristics of performing ZVS at the primary side and the secondary side, and operating with a wide voltage range without sacrificing the efficiency at normal conditions. Due to the DAB circuit can realize ZVS for the switch at the secondary side, it can be used in higher voltage output environments that require a higher efficiency. The DAB circuit has a small inductance for energy transmission, thus can be implemented through leakage inductance of a transformer. Hence the DAB circuit has a higher power density. On the computer power supply for a lower voltage output application, the DAB circuit has a greater turn-off loss than the LLC circuit. But the DAB circuit is easier to accomplish synchronous rectification than the LLC circuit.
As the LLC circuit has higher efficiency, the multi-output DC/DC converter adopted the LLC structure is a hot research topic. There are some typical structures, such as LLC+MagAmp (magnetic amplifier) (referring to FIG. 7), LLC coupling with a transformer to accomplish multiple outputs (referring to FIG. 8), LLC collocates buck and multiple independent LLCs to accomplish multiple outputs, and the like. All the circuits mentioned above have their share of advantages and drawbacks, and also have their desirable applications. The circuit of LLC+MagAmp shown in FIG. 7 has a switch 301, an LLC circuit 302, a transformer 303, a control chip 304 and a driver 305. The LLC circuit 302 provides primary output. The transformer 303 has a secondary side connecting to two magnetic amplifiers (MagAmp) 307 and an ancillary circuit such as control circuit 306 to aid output. The magnetic amplifier (MagAmp) 307 is controlled to function as a switch. The magnetic amplifier (MagAmp) 307 and the connecting ancillary circuit function as a buck circuit. Such a structure has benefits lower cost, more accurate adjustment for the output of each circuit, simpler control and the like. But it also has its share of drawbacks, such as: 1. MagAmp choke incur magnetic core loss and copper loss; 2. rectification diodes have conduction loss and reverse recovery loss; 3. synchronous rectification of the LLC circuit is greatly affected by MagAmp circuit; 4. its operation frequency is limited because of inherent dead-time effect of the MagAmp circuit. Hence the LLC+MagAmp circuit cannot achieve a higher efficiency.
FIG. 8 shows the LLC coupling with a transformer to accomplish multiple outputs. Its circuit structure includes an LLC circuit 302 connecting to a plurality of switches 301 and a transformer 303 that are the same as the conventional technique previously discussed. But it has a weighted voltage control circuit 92 and a coupling element 93 to regulate the duty period of a driver 305. Such a structure has cross regulation problem. First, the ratio of the coil number of the secondary winding is not totally equal to the output voltage ratio. Hence regulation accuracy is affected. Second, when the primary output is only controlled, the ancillary output is regulated merely by coupling that results in a lower accuracy. A weighted control can be adopted to provide improvement. But the weighted control has the error shared by two line outputs. This results in not accurate regulation of either line. Hence such a structure is applicable only in an environment where strict voltage stabilization is not required. In order to improve the cross regulation, multiple independent LLC circuits may be adopted to achieve a higher efficiency and more accurate regulation. But the cost and size are greater. Another alternative is using the LLC circuit as the primary output circuit and has a buck converter at the rear end connecting to the primary output circuit to form a secondary output circuit. Such an approach provides accurate regulation for each output circuit, but total efficiency is lower.